The targeting of chemotherapeutic agents to tumor cells is one of the most heavily investigated areas in biomedical research today. Although effective antineoplastic agents have been and continue to be discovered, there remains the immense problem of targeting these highly toxic agents specifically to tumor cells so that they do not also kill normal somatic cells and thereby cause permanent damage to vital organs or even death. Indeed, one measure of the clinical usefulness of any antineoplastic agent is its therapeutic index: the ratio of the median lethal dose (LD.sub.50) to the median effective dose (ED.sub.50) of the drug. With some cancer therapeutics this ratio is close to one, indicating a high level of toxic side effects to the patient.